The present invention relates to a nontrack-bound vehicle with arbitrarily steerable front wheels and an additional rear-wheel steering system that operates automatically or can be switched to automatic operation and controls or regulates the steering angle of the rear wheels as a function of the steering angle of the front wheels and of further parameters such as the vehicle speed or the distance traveled, the additional rear-wheel steering system switching in a parameter-dependent manner between a drive-away mode and a normal mode.
Generally, normal road vehicles with arbitrarily steerable front wheels and nonsteerable rear wheels have a short wheelbase in comparison with the length of the vehicle. Mention may be made here of large buses. This type of construction considerably facilitates steering maneuvers when traveling along narrow and winding routes, in old towns for instance. On the other hand, a long wheelbase is advantageous in regard to directional stability of the vehicle at higher speeds. In the case of a long wheelbase, the wheel housings can furthermore be placed well toward the front and rear, allowing better use to be made of the space within the vehicle and, in addition, an improvement in the use of axle loads to be achieved.
When designing a vehicle, an optimum compromise must therefore be found between easy maneuverability when steering in confined spaces and directional stability at high speed.
In the case of vehicles of the type described above, ease of handling on narrow and winding routes can be achieved even with a (very) long wheelbase and correspondingly good directional stability at high speed. Owing to the additional rear-wheel steering system, the rear wheels are actuated in the opposite direction to a steering movement of the front wheels, and the vehicle""s body thus moves relative to the underlying surface when cornering as though the vehicle had a significantly shorter wheelbase and nonsteerable rear wheels. This concept is described in German Published Patent Application No. 39 29 994, for example.
If a vehicle of this kind has a relatively long rear overlap behind the rear axle, this vehicle may get into a position close to a fixed obstacle extending parallel to the longitudinal direction of the vehicle. It is difficult for the driver to move the vehicle out of this position and away from the obstacle when driving on. This is because the front wheels must then only be deflected slightly out of their straight-ahead position in the direction away from the wall as otherwise the rear will swing out excessively in the direction of the obstacle owing to the oppositely deflected rear wheels and will collide with it. This problem occurs, for example, when the vehicle is parked close to a loading ramp parallel to the vehicle for loading or unloading and has then to be moved away from it again after the loading operation. The same problem arises, for example, with a bus that drives into a halt and there stops parallel and close to a curb. If the driver then turns the front wheels sharply in a direction away from the curb, the rear wheels will ride over the curb when subsequently starting off. This would be an unacceptable handling response from the point of view of the comfort and safety of the passengers and of passers-by.
As described in German Published Patent Application No. 41 40 124, this problem can be solved by providing vehicles that have a long rear overhang with an additional rear-wheel steering system that can be controlled in such a way that, when cornering, a point on the rear of the vehicle follows the curve of the path of an analogous point on the front of the vehicle. The result is that, as they leave a vehicle position close to an obstacle or a wall, the rear wheels are initially deflected in the same direction as the front wheels, the steering of which is actuated by the driver. This allows the driver to steer the front wheels in almost any desired manner in a direction away from the obstacle or wall.
These problems are also addressed in German Published Patent Application No. 35 06 048, which describes a method of steering for an all-wheel steering system in which the rear of the vehicle is prevented from swinging out excessively towards the outside of a bend when the front wheels are turned sharply at low speed in order, for example, to make it easier for the driver to turn straight out of a narrow thoroughfare without the risk of a collision between the rear of the vehicle and the boundaries of the thoroughfare.
In this context, German Published Patent Application No. 44 13 413 describes that, in the case of vehicles with a wheelbase that is long in comparison with their length and with steerable rear wheels, the rear-wheel steering can be set automatically or manually to a particular operating mode in which, during a driving phase following a halt or at a speed below a low minimum vehicle speed and given arbitrary deflection of the front wheels out of a straight-ahead position or a position adjacent thereto, the rear wheel that is on the outside of the bend initially follows a path that connects the front- and rear-wheel contact areas defined during a halt or at a speed below the minimum speed approximately in a straight line on the side of the vehicle that is on the outside of the curve or runs on that side of a straight line connecting the said wheel contact areas that is on the inside of the curve. On the one hand, this measure ensures that, when leaving a halt in a direction away from a curb, the rear wheels do not run over the curb at least when the driver has by his steering maneuvers avoided running over the curb with the front wheels, as is customary and easily possible. On the other hand, a kinematic behaviour similar to that with a vehicle that has a shorter wheelbase and nonsteerable rear wheels is achieved even during this special operating mode.
German Published Patent Application No. 43 41 636 describes a vehicle in which the adjustment of the rear wheels when driving away and steering away from the straight-ahead driving position follows the steering movement of the front wheels with a time delay. A steering characteristic of this kind is intended to avoid a situation where a corner of the rear of the vehicle on the outside of a bend travels over an obstacle, e.g., a curb, disposed to the side of the vehicle when driving away.
It is an object of the present invention to improve the maneuverability of a vehicle of the type described above and to reduce the swinging out of the rear when driving away.
According to the present invention, the above and other beneficial objects are achieved by virtue of the fact that, in the drive-away mode, the steering angle of the rear wheels is controlled or regulated so that the rear wheels are not adjusted beyond a steering angle, referred to below as the halt steering angle, assumed when the vehicle last halted.
The present invention is based on the principle that the rear-wheel steering angles with which the vehicle can drive through a halt are uncritical for the passengers, passers-by and for the vehicle. Accordingly, swinging out of the rear of the vehicle that takes place at these rear-wheel steering angles is not dangerous, even if the vehicle""s progress is interrupted by a halt and it continues with a drive-away process. In other words, if the vehicle travels into a halt on a curved track, i.e., with its wheels turned to a greater or lesser extent, it can be assumed that the entire curve can be traveled with the steering angles that occur during this process and that the rear can likewise travel through this curve of the halt by following the front. This is entirely irrespective of whether the vehicle leaves the halt forwardly or backwardly.
The control of the steering in accordance with the present invention ensures that, at steering angles up to the halt steering angle, the rear wheels have the same dependence on the steering angle of the front wheels in the drive-away mode as in the normal mode. The halt steering angle for the rear wheels simulates a steering stop beyond which the rear wheels cannot go during further steering movement at the front wheels. At front-wheel steering angles that would produce a rear-wheel steering angle that goes beyond the halt steering angle in the normal mode, the rear wheels remain constantly at the halt steering angle.
In a first extreme case, in which the halt is approached at the maximum steering angle, the drive-away mode is accordingly identical with the normal mode since then the halt steering angle coincides with the maximum rear-wheel steering angle that can be set in the normal mode. In another extreme case, when the vehicle approaches a halt substantially tangentially, the halt steering angle has a minimum value, in particular the value zero, with the result that the drive-away mode then corresponds substantially to operation of a vehicle without rear-wheel steering.
The steering system according to the present invention manages without complex calculations of the course of the rear of the vehicle and may thus be implemented with a relatively low outlay. The swinging out of the rear of the vehicle is nevertheless reduced and the vehicle""s maneuverability is improved.
In one example embodiment of the vehicle according to the present invention, the additional rear-wheel steering system detects when the vehicle is halting from the fact that a predetermined vehicle limiting speed is undershot. This is of particular significance, for example, when the vehicle has to travel very close to a wall, which generally should be performed at a relatively low speed. If the system did not switch to the drive-away mode in this case, a sharp turn of the steering away from the wall would inevitably lead to a collision between the wall and the rear of the vehicle.
The additional rear-wheel steering system may switch from the drive-away mode to the normal mode when the rear wheels assume a steering angle that is smaller than the halt steering angle. As an additional parameter to be satisfied, it may be stipulated, for example, that the rear wheels should approximately reach the point at which the front wheels were positioned during the last halt. As an alternative or additional measure, it may furthermore be stipulated that the vehicle should exceed a predetermined limiting speed. The switch from the drive-away mode to the normal mode therefore occurs completely unnoticed by the driver, thus ensuring that no unexpected steering movements can occur. In addition to the parameter-dependent switch, a corresponding switch in the cockpit of the vehicle may be actuated by the driver to manually activate and deactivate the drive-away mode.
According to another example embodiment of the vehicle according to the present invention, the halt steering angle may be determined both when halting in forward motion and when halting in reverse and may be maintained in the subsequent drive-away mode both when traveling forward and when traveling backward. This measure ensures that the vehicle has no problem in leaving a curved track (e.g., a parking lot in the form of a cul-de-sac), which it has been able to enter forwardly or backwardly, by traveling in the opposite direction.
In the vehicle according to the present invention, only the absolute value of the halt steering angle may be determined, thus allowing steering movements to the left and to the right only up to the absolute value of the halt steering angle. This measure has the effect, for example, that a vehicle that comes to a halt while traveling straight ahead behaves like a vehicle without steered rear wheels in the subsequent drive-away mode.
It should be appreciated that the features mentioned above and those that will be mentioned below may be employed not only in the respectively indicated combination but also in other combinations or in isolation without departing from the scope of the present invention.